1. Field of the Invention
This invention relates to a method and apparatus for comminuting coal particles, by themselves or in mixture with a sorbent, in a coal burning boiler and more particularly to such a method and apparatus in which the larger particles are recycled and made finer for more efficient burning and flame scrubbing by sorbent particles to effect reduction of SO.sub.x and NO.sub.x.
2. Description of the Related Art
In my U.S. Pat. No. 5,695,130 issued Dec. 9, 1997, the use of fast rotating screens with wide mesh openings(4-10 mesh) for comminuting solid particles of material such as coal and limestone is described. This end result is principally achieved by generating vortex grinding zones by means of the rotating screens into which the particles of the solid are driven by an upwardly moving gas stream. In my patent, the use of effective flame scrubbing for So.sub.x and NO.sub.x reduction by calcium carbide (CaC.sub.2) is described, using a flame injection of micronized lime sorbent or a co-burning of micronized coal and micronized limestone.
In pulverized coal burning boilers, that burn coal of 70%-200 mesh, the larger particles in the coal(30% at 30-120 mesh) do not burn efficiently in the main combustion zone of the boiler, thereby resulting in heat losses and fouling of the boiler walls and convection passages. The coal for use in such boilers is generally prepared in roller mills which produce coal of 70%-200 mesh size from 2" lump coal feed. The smaller the particle size, the higher the combustion rate, many larger size particles escaping complete or even partial combustion in the main boiler combustion zone. When using low NO.sub.x burners that cause a staged combustion of the coal for effecting lower levels of NO.sub.x formation, a reducing atmosphere containing carbon monoxide is formed which damages the heat exchanger tubes, and if the carbon monoxide escapes the burnout zone of the boiler, heat loss and flue gas emission problems occur. Larger fuel particles escape unburned, lowering the boiler's thermal capacity. The fly ash produced with low NO.sub.x burners contains a high percentage (4% to 20%) of unburned coal, thereby causing loss of fuel, lower thermal yield in the boiler and a contaminated fly ash of lower economic value (high loss on ignition). Commercially available, such roller mills include the Raymond Roller Mill and the Babcock and Wilcox MPS Pulverizer.
Such prior art pulverized coal burning boilers are described, for example, in U.S. Pat. No. 5,343,830 issued Sep. 6, 1994 to Alexander, et al.
Micronized sorbents (limestone, dolomite) may be used for flame scrubbing the in situ formed SO.sub.x and NO.sub.x in the boiler, utilizing micronized or pulverized coal as a fuel. In circulating fluidized bed boilers, the fuel and sorbent may be in the form of pulverized particles or lump solids.